Implementation of Selective Emitter for Industrial-Sized ... - IEEE Xplore

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE JOURNAL OF PHOTOVOLTAICS

1

Implementation of Selective Emitter for Industrial-Sized PERCs Using Wet Chemical Etch-Back Process Supawan Joonwichien, Satoshi Utsunomiya, Yasuhiro Kida, Masaaki Moriya, Katsuhiko Shirasawa, and Hidetaka Takato

Abstract—This paper introduces a selective phosphorus emitter formed by screen-printed resist masking combined with a wet chemical etch-back process for an industrial-sized passivated emitter and rear cell (PERC). Applying the selective emitter (SE) concept is expected to decrease the recombination losses at the front surface of the PERC cell. With the SE structure, we observed an increase in the open-circuit voltage (Vo c ) and short-circuit current density (Jsc ), but a decrease in the fill factor (FF), compared with homogeneous emitter cells. The sheet resistance (Rsh e e t ) of the lightly doped emitter (n+ -emitter) by the etch-back process had a considerable impact on the Vo c and Jsc , which we attributed to the reduced emitter saturation current density (J0e ) caused by a reduction in the Auger recombination in the n+ -emitter. The diminished FF was owing to the higher Rsh e e t , resulting in an increased series resistance of the cell. These results suggest that an SE structure made by the wet etch-back process is a very promising technology to improve the conversion efficiency of industrial-sized PERC solar cells. Index Terms—Passivated emitter and rear solar cell (PERC), PC1D, selective emitter (SE), silicon solar cells, wet chemical etchback process.

I. INTRODUCTION ASSIVATED emitter and rear cell (PERC) structures using p-type silicon are widely used in the crystalline silicon photovoltaics currently being introduced into mass production. A recent study by Saint-Cast et al. [1] explored the loss analysis of PERC cells fabricated in the PV-TEC pilot line [2] of the Fraunhofer Institute for Solar Energy Systems, wherein only open-circuit conditions are considered. More than half of the recombination occurs in the emitter and under the front contacts, while the second biggest loss occurs on the rear surface at the passivation and at the local aluminum-back surface field. Detailed information on the loss analysis of PERC fabricated

P

Manuscript received December 27, 2017; revised January 30, 2018; accepted February 9, 2018. This work was supported by The New Energy and Industrial Technology Development Organization of Japan. (Corresponding author: Supawan Joonwichien.) The authors are with the Fukushima Renewable Energy Institute, National Institute of Advanced Industrial Science and Technology, Koriyama 963-0298, Japan (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]. jp; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JPHOTOV.2018.2806838

in a manner similar to industrial production has been published in [1]. Recombination at the emitter and front metal grid could be reduced by the implementation of a selective emitter (SE) [3], [4]. The concept of an SE structure is based on the removal of the highly doped layers in the areas not intended for metallization. For p-type solar cells, the lightly doped emitter areas (n+ ) lead to a reduced Auger recombination. A lightly doped region showing high sheet resistance ensures a better blue response, resulting in high internal quantum efficiency. There is, however, a high contact resistance and thus high series resistance (Rs) [5]. By contrast, the heavily doped emitter areas (n++ ) underlying the contact regions reduce the contact resistance, but the carrier lifetime easily degrades owing to the enhanced Auger recombination and Shockley–Read–Hall recombination [6], [7]. Several different processes for SEs have been introduced for use in crystalline silicon solar cells, including the PERC family [8]. These processes include etch-back emitters [9]–[14], laser-doped SEs [15], [16], screen printing of a phosphorus-dopant paste [17], and so on. We are interested in the etch-back process, in which, the process was first introduced by Zerga et al. [9]. The etchback process allows a well-controlled reduction of the emitter surface concentration and a morphology-preserving of the pyramid surface structure for maintaining a high short-circuit current density (Jsc ). Especially, this process can be easily integrated into existing large-scale production lines. In this study, we have demonstrated the development of an SE technology in industrial-sized PERC processing. The main aim was to reduce the electrical losses of the phosphorus emitter, which have been reported to be major factors limiting the performance of standard PERCs. To produce an SE instead of a homogeneous emitter, a process of screen-printed resist masking combined with wet chemical etch-back was introduced into the standard p-type PERC process. The resulting SE structure greatly impacted the performance of the PERC cell, showing an increase in the open-circuit voltage (Vo c ) and the Jsc , which can be attributed to reduced emitter saturation current density (J0e ) in the n+ regions. II. EXPERIMENTAL SETUP Two types of PERC structures, standard PERC [see Fig. 1(a)] and standard PERC including an SE structure [see Fig. 1(b)], were fabricated using an industrially proven process and

2156-3381 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 2

IEEE JOURNAL OF PHOTOVOLTAICS

Fig. 1. PERC cell with (a) homogeneous emitter structure and (b) selective emitter structure.

stack of ozone atomic-layer-deposited 10-nm aluminum oxide (AlOx ) films and plasma-enhanced chemical vapor deposition (PECVD) 190-nm-thick silicon nitride (SiNy ) with refractive index (n) of 2.1, and the front side was coated with 80-nm PECVDSiNy (n = 2.1). The rear passivation is locally removed by laser ablation in order to form line-shaped rear contacts with a local contact opening linewidth of 76 μm and a pitch of 1 mm. Postdeposition annealing at 450 °C for 30 min was performed in order to obtain a high level of surface passivation [18], [19], followed by a chemical treatment wherein the PERCs were dipped into diluted potassium hydroxide and HF solutions to remove residual laser damage. Following this, the rear sides of the PERCs were metalized with a commercial Al PERC paste. Note that the main ingredients of the Al paste were Al powder, binders, glass frit, and solvent. Finally, top-side Ag printing was applied, and the finished PERCs were fired at a set temperature. B. Selective Emitter PERC Cell

Fig. 2. study.

Schematic of the process flow for the PERC cells fabricated in this

scale-up equipment at the National Institute of Advanced Industrial Science and Technology. The PERC processes with homogeneous emitter (reference) and SE are displayed in Fig. 2 and explained in Sections II-A and II-B, respectively. A. Homogenous Emitter PERC Cell (Reference Cell) The homogeneous emitter PERCs (reference cells) were fabricated on industrial-sized (156 mm × 156 mm) 2.0–2.2 Ω-cm p-type Czochralski (Cz) monocrystalline silicon wafers with an initial thickness of 200 μm. The wafers were cleaned and textured in the KOH solution to produce random pyramid surfaces. For cells with a homogeneous emitter, the highly doped emitter (n++ ) with a sheet resistance (Rsheet ) of approximately 95 Ω/sq was formed in an industrial tube furnace using POCl3 as a precursor gas, followed by the removal of phosphosilicate glass (PSG) layers. The rear-side doped layer was subsequently removed by chemical etching solution mixture consisting of hydrofluoric acid (HF) and nitric acid (HNO3 ), including the random pyramid texture on the rear of the wafers. After thorough cleaning, the rear side was passivated using a

The fabrication of the heterogeneous emitter PERCs was similar to that of the homogeneous emitter PERCs, except for the initial Rsheet of the n++ region and an additional processing sequence for SE formation (see Fig. 2). An n++ region with Rsheet of 60 Ω/sq was initially formed, followed by edge isolation. The screen-printed acid resist masking was then printed on the areas intended for front Ag metallization, aiming to protect the n++ regions during the wet chemical etch-back process. This etch-back process can be done by immersing the samples into an etching solution to obtain selectively etched-back regions (lightly doped emitter, n+ ). This etching solution for etched-back process was developed by Nippon Kasei Chemical Company Limited and the ingredient mainly consists of HF and HNO3 solutions. The final Rsheet of n++ regions depends on the etching times. The dependence of the surface morphology on the etching times of textured silicon using etching solution from the same supplier is reported in [20]. Fig. 3 presents the doping profile of heavily doped emitters, measured by SIMS showing the phosphorus surface concentration of about 7 × 1020 atoms/cm3 with a junction depth of ∼0.47 μm. The immersion times of 30 and 35 s resulted in the removal of heavily doped emitters by 47 and 54 nm, and resulted in Rsheet values of 125 and 160 Ω/sq, respectively. The acid-resistant barrier layer was subsequently stripped by soaking in isopropyl alcohol and further cleaning. From here, the heterogeneous emitter PERC cells were fabricated using the same process as that for the homogeneous PERC cells. C. J0e Investigation In order to study the emitter quality of a cell process, symmetrical PECVD-SiNy /Si/PECVD-SiNy structures were also fabricated on the textured and phosphorus-diffused Si wafers, with Rsheet values of 95, 125, and 160 Ω/sq, respectively. A schematic layout of the sample structure is shown in Fig. 4. An 80-nm SiNy film with n of 2.1 was deposited on both sides of the symmetric samples. The emitter quality was quantified by the J0e of the n+ -emitter, which can be extracted from lifetime measurements of the symmetric samples using quasi-steady-state

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. JOONWICHIEN et al.: IMPLEMENTATION OF SELECTIVE EMITTER FOR INDUSTRIAL-SIZED PERCS USING WET CHEMICAL ETCH-BACK

3

Fig. 5. Correlation between Jsc , V o c , FF, and η values and sheet resistance of n+ regions for PERCs.

III. RESULTS AND DISCUSSION A. Influence of Applied Selective Emitter Structure

Fig. 3. SIMS profile of a 60 Ω/sq phosphorus emitter. The removal of emitters by 47 nm and 54 nm by etch-back process leads to the surface concentrations of 2.5 × 1020 atoms/cm3 and 1.8 × 1020 atoms/cm3 , respectively.

Fig. 4.

Symmetrical sample structure scheme for J0e measurement.

photoconductance measurements (Sinton, WCT-120) [21] with the high injection level method proposed by Kane and Swanson [22] 1 1 (Na + Δn) 1 − = + (J0e,front + J0e,rear ) τeﬀ τAuger τSRH qn2i W (1) where τeﬀ , τAuger , and τSRH are the measured effective lifetime, the intrinsic Auger lifetime, and the defect-related bulk lifetime, respectively [23], W is the sample thickness, Δn is the excess carrier density (Δn = 3 × 1015 cm−3 ), q is the elementary charge, ni is the intrinsic carrier concentration of c-Si (ni = 8.6 × 109 cm−3 ) [24], and Na is the bulk doping concentration (acceptor for p-type).

Cell parameters of the industrial PERC cells (SE and reference cells) were measured under standard testing conditions with a cell area of 239 mm2 . The correlation between Jsc , Vo c , fill factor (FF), and conversion efficiency (η) values and Rsheet of n+ regions is shown in Fig. 5. The SE structure greatly impacted the performance of the PERC cell, showing an increase in the Jsc of 0.3 mA/cm2 , and an increase in the Vo c of 10 mV, but a decrease in the FF of 1.5%, compared with the homogeneous emitter cells. As measured on textured and PECVDSiNy -passivated surfaces (see Fig. 6), the J0e diminished with increasing Rsheet of the n+ -emitter, and was strongly reduced to values in the range of 80–160 fA/cm2 only if the samples were fired. Note that a lower J0e could be achieved on planar wafer surfaces than on alkaline textured surfaces. This can be understood by considering the surface area and crystal orientation. The recombination associated with textured surface is due to, such as, a spatially nonuniform diffusion profile [25], additional defects induced by film stress [26], and a larger surface area [27]. As shown in Fig. 6(b) and (c), a large decrease in J0e by a factor of two or three after annealing and firing can be explained by the SiNy /Si interfaces being passivated by hydrogen at the dangling bond defects. The SiNy film is well known as a reservoir of hydrogen that ensures the stability of the interface. In Fig. 6(c), J0e decreases when the surface is lightly doped and well passivated, because the Shockley–Read–Hall recombination at the surface and the Auger recombination in the emitter are both low. In the case of the 160 Ω/sq emitter samples, approximately ∼54 nm of the highly doped emitters were removed resulted in a reduced J0e during etch-back process. By contrast, the higher J0e of the heavily doped samples indicates that Auger recombination is the dominant recombination mechanism. Table I shows



Fig. 7. Reflectance and measured and simulated IQE by PC1D curves of reference cell with a sheet resistance of 95 Ω/sq.

resistance between two fingers would lead to a rising Rs , thereby reducing FF by 1%–2%. In this study, the SE PERC did not reach its potential because of a diminished FF and an increased Rs . From our calculation, to maintain the FF of more than 80%, the finger pitch should be reduced to be ∼0.78 mm for SE PERC with Rsheet of n+ regions of 160 Ω/sq. This means that the fine line fingers front metallization technology is required. B. Influence of Rsheet of Doped Regions Fig. 6. Recombination parameter J0e as a function of the R sh e e t for (a) PECVD-SiNy -passivated samples, (b) 450 °C for 30 min annealed SiNy passivated samples shown in Fig. 6(a), and (c) fired samples shown in Fig. 6(b). Note that the J0e of n+ + regions (∼60 Ω/sq) for fired sample was approximately 316 fA/cm2 . TABLE I TLM MEASUREMENTS Parameter

Actual R sh e e t (Ω/sq) Finger pitch (mm) Resistance between two fingers; finger length of 1.4 mm (Ω) Contact resistivity (mΩ.cm2 )

Ref. PERC 95 Ω/sq

SE PERC 125 Ω/sq

SE PERC 160 Ω/sq

118 1.4 17.8

159 1.4 22.5

223 1.4 31.5

2.42

0.24

0.32

the contact resistance results of the finished cells, measured by transmission line method (TLM). It was found that the actual Rsheet values of the finished cells increased, presumably due to the chemical treatment processes, i.e., HF treatment, and PSG removal during cell fabrication. They were observed to be 118, 159, and 223 Ω/sq, which resulted in the resistance between two fingers of 17.8, 22.5, and 31.5 Ω, respectively. The increase in

As illustrated in Fig. 5, the results show significant improvements in the Jsc and Vo c for the SE PERC with a lightly doped emitter at the areas not intended for metallization. However, the procedure that produces the final Rsheet is different, that is, the 95 Ω/sq emitter samples (the reference cell included J0e samples) were obtained just after phosphorus diffusion, while other samples with higher Rsheet were achieved after etching in chemical solution. In order to compare and determine the influence of Rsheet on the cell performance, a simulation was carried out using the PC1D program [28]. To ensure a realistic simulation, the experimental data were used to calculate the Vo c with a variation of Rsheet emitter by fitting the calculation data with measurements of the internal quantum efficiency (IQE) and reflectance of the reference cell. Fig. 7 presents the measured and simulated IQE by PC1D of the reference cell with an Rsheet of 95 Ω/sq. The basic input parameters for PC1D are shown in Table II. The simulated IQE was in good agreement with the measured curve, as shown in Fig. 7. To further estimate the Vo c from the fitting data shown in Fig. 7, we varied the Rsheet from 60 to 160 Ω/sq; the dependences of the simulated Vo c as a function of the Rsheet of the emitter are plotted in Fig. 8. Note that the initial Rsheet of samples after phosphorus diffusion for SE PERC was 60 Ω/sq, which later increased to 125 and 160 Ω/sq, respectively, after the wet chemical etch-back process in order to selectively obtain the lightly doped emitter. As a result, when increasing Rsheet


5

TABLE II PC1D FITTING PARAMETERS Model parameter Device area Bulk lifetime Bulk resistivity Bulk thickness 1st front diffusion 2nd front diffusion Junction depth Front surface recombination velocity Rear surface recombination velocity

Unit 1 260 2 170 7 × 1020 2 × 1019 0.46 126 65

cm2 μs Ω-cm μm cm−3 cm−3 μm cm/s cm/s

Fig. 9. IQE and reflectance of the reference cells and the SE PERC. The inserted SEM images show the front cell surfaces of reference PERC and SE PERC.

Fig. 8. Dependences of the simulated V o c by PC1D and the measured V o c as a function of the R sh e e t of emitter.

from 60 to 160 Ω/sq, the Vo c increased by 13 mV from its original value. This confirms that the Rsheet of n+ regions in the SE PERC greatly impacted the cell parameters, which was attributed to the reduced J0e in the n+ regions because of the reduced Auger recombination. As shown in Fig. 8, the actual measured Vo c values from experiments with the reference cells were much higher than the simulated Vo c obtained by PC1D. For PC1D analysis, the input parameters of the recombination loss in the local back surface field and metal contacts are considered by assuming a rear surface recombination velocity as given in Table II. According to the loss analysis of PERCs reported by Saint-Cast et al. [1], approximately 14% and ∼6.5% of the recombination occurs in the emitter and under the front contacts. Therefore, the improvement of the emitter by an SE structure would lead to low recombination at the front, and enhanced Vo c and η. C. Internal Quantum Efficiency As the short-wavelength spectral response of a cell strongly reflects the recombination at the front, IQE measurements of the solar cells were used to compare the absorption coefficients of

Si for blue light (short-wavelength photons). Fig. 9 shows the IQE data for the reference cell and the SE cell. It should be noted that the reference cell samples for the IQE data in Figs. 7 and 9 are different. The IQE of the SE cell shows an improvement for short-wavelength photons, which indicates a reduction in Auger recombination, as compared with the reference cell. As can be seen in Fig. 5, the small difference between the IQEs for blue light is due to the Jsc value, which increased by only 0.3 mA/cm2 for the SE cell with Rsheet of 125 Ω/sq in the n+ regions. Note that only minor portions of the tips of the pyramids were etched away during an etch-back from 60 to 160 Ω/sq as shown in the inserted scanning electron microscope (SEM) images in Fig. 9. This would result in the slight increase in reflectance, thereby slightly reducing the Jsc for SE PERCs. In addition, the main Jsc losses for PERCs are the rear recombination and absorption [1], which could be reduced by improving the passivation scheme. Fig. 10 presents an optical microscope image of the finished PERC cell showing an Ag finger aligned directly on the n++ region, surrounded by the lightly doped n+ emitter regions. The IQE map at a single wavelength of 400 nm of the finished cell, which is shown in Fig. 10, is presented in Fig. 11. The IQE map was recorded in order to quantitatively visualize the spatial distribution of passivation quality of the n+ and n++ regions at the front of the cell. The IQE measurement was performed in steps of X = 0.1 mm and Y = 0.1 mm scale on a small area of 5 mm × 8 mm of the cell surface. The detailed principles of the IQE measurements are described elsewhere [29]. The areas shown in blue have the highest IQEs and those in red have the lowest IQEs. As can be seen in the IQE map, the n+ regions (bright blue) show good homogeneity with lower surface recombination velocities compared with those in the n++ regions (green) under the Ag metal grids (yellow). These spatial distributions obtained from the IQE map suggest room for improvement that



density (J0e ) in the n+ regions. The decline in FF was due to the higher sheet resistance in the n+ regions, resulting in higher series resistance (Rs ). These results suggest that the SE concept is a very promising technology to improve the conversion efficiency of industrial-sized PERC solar cells. ACKNOWLEDGMENT The authors would like to thank their colleagues at AIST for many contributions toward the fabrication of PERC solar cells, and for a fruitful discussion. REFERENCES

Fig. 10. Optical microscope image of the finished PERC cell with SE structure showing n+ + regions placed directly under the Ag front finger.

Fig. 11. IQE map at a single wavelength of 400 nm showing the IQE signals of heavily doped and lightly doped emitters at the front of PERC.

the range of n++ regions should be narrower for increasing ratio of unmasked/masked regions. This idea would lead to the improvement of Vo c and Jsc of the cell due to the reduction in J0e . IV. CONCLUSION We report on our attempt to improve PERC performance by introducing an SE technology using screen-printed resist masking combined with a wet chemical etch-back process. Applying the SE concept was intended to decrease the recombination losses at the front surface of the cell. Indeed, the SE structure had a considerable impact on the performance of the PERC cell, showing an increase in the open-circuit voltage (Vo c ) of 10 mV, and an increase in the short-circuit current density (Jsc ) of 0.3 mA/cm2 , but a decrease in the FF of 1.5%, compared with homogeneous emitter cells. The increased Vo c and Jsc were mainly attributed to the reduced emitter saturation current

[1] P. Saint-Cast et al., “Analysis of the losses of industrial-type PERC solar cells,” Phys. Status Solidi A, vol. 214, no. 3, 2017, Art. no. 1600708. [2] D. Biro et al., “PV-Tec: Photovoltaic technology evaluation center-design and implementation of a production research unit,” in Proc. 21st Eur. Photovolt. Sol. Energy Conf. Exhib., Dresden, Germany, 2006, pp. 621– 624. [3] A. Zerga et al., “Selective emitter formation for large-scale industrially mc-Si solar cells by hydrogen plasma and wet etching,” in Proc. 21st Eur. Photovolt. Sol. Energy Conf. Exhib., Dresden, Germany, 2006, pp. 865– 869. [4] H. Haverkamp, A. Dastgheib-Shirazi, B. Raabe, F. Book, and G. Hahn, “Minimizing the electrical losses on the front side: Development of a selective emitter process from a single diffusion,” in Proc. 33rd IEEE Photovolt. Spec. Conf., San Diego, CA, USA, 2008, pp. 430–433. [5] M. J. Kerr and A. Cuevas, “General parameterization of Auger recombination in crystalline silicon,” J. Appl. Phys., vol. 91, no. 4, pp. 2473–2480, 2002. [6] A. Rohatgi et al., “Self-aligned self-doping selective emitter for screenprinted silicon solar cells,” in Proc. 17th Eur. Photovolt. Sol. Energy Conf. Exhib., Munich, Germany, 2001, pp. 22–26. [7] B. Min, H. Wagner, A. Dastgheib-Shirazi, and P. P. Altermatt, “Limitation of industrial phosphorus-diffused emitter by SRH recombination,” Energy Procedia, vol. 55, pp. 115–120, 2014. [8] M. Green, “The passivated emitter and rear cell (PERC): From conception to mass production,” Sol. Energy Mater. Sol. Cells, vol. 143, pp. 190–197, 2015. [9] A. Zerga et al., “Selective emitter formation for large scale industrially mc-Si soalr cell by hydrogen plasma and wet etching,” in Proc. 21st Eur. Photovolt. Sol. Energy Conf. Exhib., Dresden, Germany, 2006, pp. 865– 868. [10] Y. Tao, K. Madani, E. Cho, B. Rounsaville, V. Upadhyaya, and A. Rohatgi, “High-efficiency selective boron emitter formed by wet chemical etchback for n-type screen-printed Si solar cells,” Appl. Phys. Lett., vol. 110, 2017, Art. no. 021101. [11] Y. Schiele, S. Joos, G. Hahn, and B. Terheiden, “Etch-back of p+ structures for selective boron emitters in n-type c-Si solar cells,” Energy Procedia, vol. 55, pp. 295–301, 2014. [12] A. Dastgheib-Shirazi, H. Haverkamp, B. Raabe, F. Book, and G. Hahn, “Selective emitter for industrial solar cell production: A wet chemical approach using a single side diffusion process,” in Proc. 23rd Eur. Photovolt. Sol. Energy Conf. Exhib., Valencia, Spain, 2008, pp. 1197–1199. [13] F. Book, A. Dastgheib-Shirazi, B. Raabe, H. Haverkamp, G. Hahn, and P. Grabitz, “Detailed analysis of high sheet resistance emitters for selectively doped silicon solar cells,” in Proc. 24th Eur. Photovolt. Sol. Energy Conf. Exhib., Hamburg, Germany, 2009, pp. 1719–1722. [14] G. Hahn, “Status of selective emitter technology,” in Proc. 25th Eur. Photovolt. Sol. Energy Conf. Exhib., Valencia, Spain, 2010, pp. 1091– 1096. [15] R. S. Davidsen et al., “Black silicon laser-doped selective emitter solar cell with 18.1% efficiency,” Sol. Energy Mater. Sol. Cells, vol. 144, pp. 740– 747, 2016. [16] K. Hirata, T. Saitoh, A. Ogane, E. Sugimura, and T. Fuyuki, “Selective emitter formation by laser doping for phosphorous-doped n-type silicon solar cells,” Appl. Phys. Express, vol. 5, 2012, Art. no. 016501. [17] S. Zhong, W. Shen, F. Liu, and X. Li, “Mass production of high efficiency selective emitter crystalline silicon solar cells employing phosphorus ink technology,” Sol. Energy Mater. Sol. Cells, vol. 117, pp. 483–488, 2013.


[18] B. Hoex, S. B. S. Heil, E. Langereis, M. C. M. van de Sanden, and W. M. M. Kessels, “Ultralow surface recombination of c-Si substrates passivated by plasma-assisted atomic layer deposited Al2 O3 ,” Appl. Phys. Lett., vol. 89, 2006, Art. no. 042112. [19] B. Hoex et al., “Excellent passivation of highly doped p-type Si surfaces by the negative-charge-dielectric Al2 O3 ,” Appl. Phys. Lett., vol. 91, 2007, Art. no. 112107. [20] S. Simayi, Y. Kida, K. Shirasawa, T. Suzuki, and H. Takato, “Method of removing single-side doped layer while maintaining pyramid textured surface of n-type bifacial solar cells,” IEEE J. Photovolt., vol. 7, no. 2, pp. 458–462, Mar. 2017. [21] R. A. Sinton and A. Cuevas, “Contactless determination of currentvoltage characteristics and minority-carrier lifetimes in semiconductors from quasi-steady-state photoconductance data,” App. Phys. Lett., vol. 69, pp. 2510–2512, 1996. [22] D. E. Kane and R. M. Swanson, “Measurement of the emitter saturation current by a contactless photoconductivity decay method,” in Proc. 18th IEEE Photovolt. Spec. Conf., Las Vegas, NV, USA, 1985, pp. 578–583. [23] R. A. Sinton and R. M. Swanson, “Recombination in highly injected silicon,” IEEE Trans. Electron Devices, vol. ED–34, no. 6, pp. 1380– 1389, Jun. 1987. [24] A. B. Sproul and M. A. Green, “Improved value for silicon intrinsic carrier concentration from 275 to 375 K,” J. Appl. Phys., vol. 70, pp. 846–854, 1991.

7

[25] S. W. Glunz, S. Sterk, R. Steeman, W. Warta, J. Knobloch, and W. Wettling, “Emitter dark saturation currents of high-efficiency solar cells with inverted pyramids,” in Proc. 13th Eur. Photovolt. Sol. Energy Conf. Exhib., Nice, France, 1995, pp. 409–412. [26] P. J. Cousins and J. E. Cotter, “Minimizing lifetime degradation associated with thermal oxidation of upright randomly textured silicon surfaces,” Sol. Energy Mater. Sol. Cells, vol. 90, no. 2, pp. 228–240, 2006. [27] K. R. McIntosh, L. P. Johnson, K. R. McIntosh, and L. P. Johnson, “Recombination at textured silicon surface passivated with silicon dioxide,” J. Appl. Phys., vol. 105, 2009, Art. no. 124520. [28] D. A. Clugston and P. A. Basore, “PC1D version 5: 32-bit solar cell modeling on personal computers,” in Proc. 26th IEEE Photovolt. Spec. Conf., Anaheim, CA, USA, 1997, pp. 207–210. [29] T. Mochizuki, S. Joonwichien, J. Mitchell, K. Tanahashi, K. Shirasawa, and H. Takato, “Evaluation of rear surfaces of PERC solar cells using internal quantum efficiency mapping,” in Proc. 26th Int. Photovolt. Sci. Eng. Conf., Singapore, 2016.

Authors’ photographs and biographies not available at the time of publication.